The principle of "push-broom" type scanning is shown in FIG. 1 for the case of a detector strip 1.
As the satellite carrying the strip 1 moves, the strip observes successive lines L1, L2, . . . , Ln extending perpendicularly to its displacement direction (arrow D). At any instant, an instrumentation optical system 2 forms an image of a line of the scene on a line of detectors, the strip 1 being located in the focal plane of the optical system 2 extending perpendicularly to the velocity vector of the satellite. The scene passes in front of each detector which integrates the light flux during exposure time and transforms it into a proportional electrical charge.
FIG. 2 shows a conventional system for processing images taken in this way.
In outline, the processing system comprises a unit 3 for processing and amplifying the output from the detectors of the strip 1, an analog-to-digital encoder 4 receiving the output signal from the unit 3, means 5 for transmitting the digital images taken in this way from the satellite to the ground, and a unit 6 on the ground for reconstituting images.
The unit 3 comprises, in particular, a shift register into which the information integrated and stored in each detector of the strip 1 in the form of charge is transferred at the end of the exposure time. This register then transfers charge electronically, converting it into a succession of voltages proportional to points of light flux as received and integrated.
The unit 6 on the ground reconstitutes images in particular by implementing deconvolution processing to compensate for instrument defects, and also, where appropriate, interpolation processing for reconstituting certain pixels of the image.
Usually, the acquisition time between two successive lines, or "sampling time", is such that the point on the ground situated vertically below the satellite is displaced through a distance equal to the dimensions of an individual detector as projected onto the ground.
With reference to the focal plane of the instrument, this gives rise to line and column sampling frequencies that are the same and equal to the reciprocal of the size of an individual detector in the focal plane.
However, this approach does not take account of the instrument modulation transfer function (MTF).
It is recalled that the modulation transfer function of an optical system is a function in frequency space representing the suitability of the system for transmitting various frequencies. It is characteristic of the reproduction of contrast in the scene by the system.
For push-broom type acquisition, the modulation transfer function depends mainly on the optics of the system, on the smearing effect (reduction in contrast due to motion), and on the detectors.
Observation systems are characterized by the cutoff frequency fc above which the MTF is negligible. A cutoff frequency can be associated with each of the effects contributing to the overall modulation transfer function (optical system, smearing, and integration on the photosensitive zone), the overall cutoff frequency being the smallest of the three above values.
The modulation transfer function associated with integration in the photosensitive zone cancels at a frequency equal to the reciprocal of the size of an individual detector which constitutes a first approximation to the corresponding cutoff frequency.
It is the integration effect on the photosensitive zone which generally determines the overall cutoff frequency.
For conventional acquisition where the sampling frequency fe is equal to the cutoff frequency fc, the Shannon condition (fe.gtoreq.2.fc) is not satisfied and this gives rise to a high degree of spectrum aliasing which introduces artifacts and makes any attempt at deconvolution or at interpolation difficult.